The instant invention pertains to apparatus for holding pipe in a vertical position and more particularly to such apparatus which does so using a set of pipe slips.
In the drilling of oil and gas wells, it is necessary to thread together numerous lengths of pipe to form a drill string which rotates a bit at the bottom thereof. During drilling, at least some of the weight of the pipe string is supported by a block and tackle arrangement suspended from a mast which extends upwardly from the floor of the drilling rig.
When it is necessary to add an additional pipe to the top of the drill string, rotary action of the drill string is stopped and it is suspended at the floor of the drilling rig while an additional pipe is threadably connected to the uppermost pipe in the drill string. The drill string is typically suspended by a so-called spider which is mounted on the floor of the drilling rig and through which the drill string extends. The spider is essentially a bowl having a hole at the lower end thereof through which the pipe at the upper end of the drill string extends. The bowl includes a tapered surface and a plurality of arcuate slip segments which also include tapered surfaces that abut against and are slidable along the tapered surface in the bowl. Each slip segment includes gripping elements directed toward the pipe. When the segments are received in the bowl about the circumference of the pipe and the pipe is lowered, camming action between the segments and the bowl jams the segments into the pipe thus suspending it from the spider.
When the pipe is so suspended, an additional joint of pipe is threadably engaged with the uppermost pipe on the drill string, the slip segments are removed, and rotary motion is again applied to the drill string to continue drilling.
During drilling, it may be necessary to remove the drill string to change the bit, to add casing to a portion of the well bore, or for other reasons. When removing the drill string, rotary motion is stopped and the drill string is suspended in the spider. Thereafter, an elevator which is suspended from the traveling block in the block and tackle arrangement is used to grip the pipe just above the spider and the spider slip segments are removed. The traveling block is raised, the spider slips are reset and the stand of pipe extending above the drilling rig floor is unthreaded and removed. Thereafter, the elevator grasps the pipe extending from the spider, the spider slip segments are again removed, and the traveling block again raised. This process is repeated until the drill string is removed from the bore.
The elevator, like the spider, includes arcuate slip segments which are received in a bowl which tends to cam the segments into the pipe in order to provide a tight gripping action.
Several problems exist with the prior art spiders and elevators. Because of slight amounts of uneveness which may exist on the radially outer surface of a drill pipe, it is possible for one of the slip segments to abut against the pipe so that the gripping elements formed on the slip segment are not uniformly loaded. Unevenness in loading tends to concentrate forces against the pipe and in a severe case may amount to point loading. When supporting a drill string which may be several miles in length, such loading can cause pipe failure which permits the drill string to fall in the bore.
Another problem with prior art elevators and spiders is that there is a substantial gap between adjacent slip segments when such are received in the bowl to support the pipe. Thus, between adjacent slip segments there is a vertical gap against which gripping elements are not abutted. When supporting several miles of drill string, the radially inward pressure of all of the gripping elements tends to cause the pipe to bulge into the verticle gaps. Such bulging can cause pipe failure.
The instant invention comprises a bowl having an upper surface which is tapered from a larger opening at the upper end thereof to a smaller opening at the lower end thereof. A first pair of substantially arcuate slip segments are receivable in the bowl. The slip segments have inwardly tapered edges which ride on the bowl inner surface when the segments are received therein for clamping a pipe as the segments are lowered into the bowl. A second pair of substantially arcuate slip segments are receivable in the bowl above the first pair. The second pair of segments have inwardly tapered edges which ride on the bowl inner surface when the second pair of segments are received therein for clamping a pipe as the second pair of segments are lowered into the bowl.
It is an object of the present invention to provide layered pipe slips which grip pipe in an improved manner while reducing the stress generated during gripping.
It is another object of the invention to provide such slips which reduce the above-described uneven loading and pipe bulging which are associated with prior art slips.